1. Field of the Invention:
This invention relates, in general, to resistance spot welding equipment and, more specifically, to resistance spot welding electrodes.
2. Description of the Prior Art:
Resistance spot welding is a process commonly used to join two thin metal sheets together. A spot weld is produced by clamping the two thin sheets of metal together under high pressure or force between two axially aligned electrodes and passing a high electrical current between the electrodes through the metal sheets. The high resistance at the interface of the two metal sheets causes heating of the sheets in an area between the contact surfaces of the electrodes. This rapid heating creates a molten zone of metal which, when cooled, forms a solid weld spot or nugget fusing the two metal sheets together.
The electrodes used to form a spot weld are typically constructed of a high electrically conductive material, such as copper. The size and shape of the electrode determines the size and shape of the spot weld. The electrodes may have an integral single piece form or may be constructed of a holder or shank portion which receives a replaceable cap or tip at one end. The interior of the electrode is hollow and is provided with coolant flow paths for removing heat generated by the high currents passing through the electrode.
The diameter of the electrode or tip, which is generally circular in form, is a critical parameter in producing spot welds of sufficient size to hold two metal sheets together. Thus, depending on the thickness of the two sheets, the electrode pressure, the welding current, etc., the diameter of the electrode is selected to produce a particular diameter weld spot sufficient to hold the two sheets together.
During repeated spot welding operations, the electrodes experience considerable deformation or wear at their contact surfaces due to the high clamping pressures and the heat generated by the current passing through each electrode. Such wear or deformation causes the contact surface of the electrode to spread out or "mushroom" which increases its diameter. The increased diameter of the electrode has adverse effects on the spot weld to be produced since less heat is generated at the metal sheet interface due to current flow through the larger contact area of the two electrodes. Thus, frequent reshaping or dressing of the electrode tip is required to return the tip to its orignal size and shape. This necessitates interruption of the welding operation which lowers production and increases labor costs since hand machining by a file is typically employed to reshape the electrode tip.
Thus, it would be desirable to provide a spot welding electrode which overcomes the problems of previously devised spot welding electrodes which require frequent reshaping or redressing. It would also be desirable to provide a spot welding electrode which substantially eliminates the need for reshaping or redressing during repeated spot welding operations.
In U.S. Pat. No. 4,476,372, the disclosure of which is hereby incorporated by reference, there is disclosed and claimed an improved electrode of the type under consideration herein, wherein the electrode tip has a plurality of concentric enlarged sections adjacent reduced sections, the outer sides of which act as cleavage surfaces for removing excess material caused by successive deformation of the enlarged sections. While the electrode is eminently useful it has been found that certain improvements therein will extend the usefulness thereof by providing for better cleavage of the outer surfaces of the enlarged sections.